The present invention relates to control of the cooling of electrical power generators, particularly generators which are hydrogen cooled.
In view of the substantial levels of heat produced in electrical generators employed in power plants, they must be actively cooled, and it is common practice to utilize hydrogen for this purpose and to remove heat from the hydrogen in hydrogen coolers. Typical hydrogen coolers are cross-flow, extended-surface heat exchange units which transfer heat from hydrogen gas to cooling water by indirect heat exchange.
When this technique is used, the cooling water is usually extracted from a river, lake or cooling tower, so that the cooling water inlet temperature undergoes at least long term variations.
Whenever the cooling water inlet temperature varies, adjustments must be made to prevent corresponding variations in the hydrogen gas cooling. Overcooling of the circulating hydrogen gas would cause significant problems in other parts of the generator, such as diminished tolerances and increased moisture condensation. Undercooling, on the other hand, will result in overheating in other parts of the generator.
It is the current practice to compensate for changes in cooling water temperature by making an offsetting variation in cooling water flow rate. However, changes in the cooling water flow rate cause other problems.
For example, it is known that a thin sediment film normally coats the inside walls of cooling water tubes in such hydrogen coolers and when the cooling water flow rate is reduced, below approximately one meter per second, the thickness of this film increases and causes corrosion and pin holes which are the sources of leaks, as well as causing an increased fouling resistance which interrupts effective heat transfer.
If, on the other hand, the cooling water flow rate increases, for example above about three meters per second, the walls of the cooling water tubes, which are usually made of a copper alloy, are subject to erosion which results in pitting. In addition, increased flow rates cause undesirable pump power losses.
While many of the problems associated with changes in cooling water flow rate could be obviated by the use of stainless steel for the cooling water tubes, copper-bearing alloys are preferred for this purpose because of their superior heat transmission properties. Moreover, stainless steel is still subject to stress corrosion. Therefore, a change in tube material would not provide a complete solution to the problems caused by cooling water flow rate variations.